Those tranny need to have the cable adjusted so it is exactly dead on, one bolt hole wrong on the cable retainer can make it out of adjustment
You need to remove the tranny pan and don't remove the bolt on the adjustment wheel and put in neutral on the shifter buttons. Ten slowly remove the male pin from the rooster comb on the valve body and see if the rooster comb moves any at all, if it does move adjust the wheel so it doesn't not move at all when you lift the pin out of that hole. Once you have that adjustment correct check it in 1st, 2nd and high gear for movement on them also
It needs to be perfect to work correctly in all four positions
You can do this, take your time and get it correct.
The reason for the RPM drop on the gear change in the upper RPM range is the manual valve in the valve body is routing pressure to the wrong spot for a clean gear change, hence the over lap and RPM drop